Concept:The change from solid to liquid is called melting, and for a pure substance it occurs at one fixed temperature known as the melting point.
Explanation:When a solid is heated, its temperature rises until it reaches the melting point.
At this stage, the heat energy supplied is absorbed as latent heat of fusion.
This energy is used to overcome the forces of attraction holding the particles in a fixed arrangement.
As a result, the temperature does not rise even though heating continues.
It remains constant until the entire solid has changed into liquid.
Only after melting is complete does further heating increase the liquid's temperature.
This shows the change of state from solid to liquid occurs at constant temperature, not at the boiling point or near the freezing point.
Answer:A. at constant temperature